37 THE SEED AND ITS GERMINATION 



The first external change is the pushing out of the 

 radicle into the soil, sometimes through the micropyle, 

 sometimes through a break in the testa, into the soil, 

 where it bends straight downwards to become the tap- 

 root, and often grows to a considerable length before 

 the shoot appears. In plants with a " fibrous " root 

 system the development of the taproot soon stops, and 

 other roots grow out from the base. 



In the development of the shoot we have to distinguish 

 two types of germination. In the first, by far the com- 

 monest type, the hypocotyl increases enormously in 

 length, thrusting, or sometimes pulling, the cotyledons, 

 with the epicotyledonary bud lying between them, out 

 of the testa (Fig. 63, A). The cotyledons are said to 

 be epigeal (raised above the soil). The cotyledons are 

 often held with a certain firmness by the testa, and 

 the base of the hypocotyl being fixed by the primary 

 root in the soil, the hypocotyl curves in elongating, 

 setting up a strain and eventually hauling the cotyledons 

 out of the testa. As soon as these are free the hypocotyl 

 straightens out, carrying the cotyledons on its summit, 

 and these expand and turn green (if they are not green 

 already), standing out on each side in a horizontal 

 or upwardly inclined position. 



This is the end of the first stage of growth of the 

 seedling, which thus becomes an autonomous green 

 plant, no longer dependent on the food in the endosperm, 

 which has been mostly absorbed by the time the 

 hypocotyl elongates. Many epigeal cotyledons are 

 thin, like the later formed foliage leaves, but in some 

 cases the fleshy cotyledons of non-endospermic seeds 

 are raised above the ground (French bean). 



Sometimes the cotyledons are not at once successfully 



1 Greek eiri, on, and yrj, earth. 



